We are currently experiencing rapid merging of fixed and mobile networks, and IMS (IP Multimedia Subsystem) appears in such environment. IMS is a global, access-independent and standards-based IP communication link and service control architecture, which makes it possible for users of common Internet protocol based terminal to use different types of multimedia services. IMS system not only provides a variety of access manners, but also provides interworking with a circuit switch domain. For a multi-mode mobile terminal locating within a packet switch domain, it can hand over a conversation from the packet switch domain to a circuit switch domain to ensure conversation quality, when it moves to an edge of or outside the network covered by the packet switch domain, or when the network covered currently by the packet switch domain becomes unavailable while it locates within the network covered by the circuit switch domain.
3GPP TS 23.216 specifies for calls anchored in IMS system a SRVCC (Single Radio Voice Call Continuity) solution between EPS (Evolved Packet System) PS (Packet Switch) access and UTRAN/GERAN (Universal Terrestrial Radio Access Network/GSM EDGE Radio Access Network) CS (Circuit Switch) access, to maintain continuities of the voice alls. In such situation, UE (User Equipment) is capable of transmitting or receiving data over only one of those two access networks at a given time.
FIG. 1 illustrates a network architecture of the SRVCC for VoIP conversation handover from E-UTRAN (Evolved UTRAN) to UTRAN/GERAN as specified in 3GPP TS 23.216.
As shown in FIG. 1, UE accesses IMS via E-UTRAN and S-GW/PDN GW. E-UTRAN is also known as LTE (Long Term Evolution), comprising several E-Nodes B responsible for radio access network part. EPS is simplified as two network elements of eNodeB and EPC by functional integration of NodeB, RNC, and CN of existing WCDMA and TD-SCDMA systems. EPC comprises MME (Mobility Management Entity) for acting as a control node with the responsibility of handling signaling of a core network, and S-GW (Serving GateWay)/PDN-GW (Packet Data Network GateWay) responsible for processing data of the core network. Wherein, a non-3GPP radio access to network may access EPC via PDN-GW, and a 3GPP radio access network may access to EPC via S-GW.
In addition, FIG. 1 further shows interfaces between network elements proposed by the specification. For example, E-UTRAN and EPC are connected via S1 (similar to Iu) interface, E-UTRANs therebetween may connect via X2 (similar to Iur) interface (not shown), and the UE and E-UTRAN are connected via LTE-Uu interface.
In the environment shown in FIG. 1, UE can decide to hand over to a circuit switch domain provided by UTRAN/GERAN when it locates at the edge of coverage of E-UTRAN or outside the covered area. In UTRAN/GERAN, UE accesses to an IMS network via a base station and an MSC (Mobile Switch Center) server.
Wherein, UTRAN is a relatively new access network for UMTS, which has became a more important access manner for UMTS, and may include NodeB, (Node B), RNC (Radio Network Controller), CN (Core Network), etc., wherein one RNC and one or more NodeBs constitute one RNS (Radio Network Subsystem); while GERAN which is a key part of GMS specified and maintained by 3GPP is also included in UMTS/GSM network, and includes a base station BS and a base station controller BSC their interfaces (for example, Ater interface, Abis interface, A interface, etc.). A network of a mobile operator generally consists of a plurality of GERANs which will be combined with UTRAN in a UMTS/GSM network.
Reference may be made with respect to TS 23.216 for detailed information on other network elements in FIG. 1 as well as communication manners between network elements or the like.
FIG. 2 illustrates a flow chart of a call of SRVCC handover from E-UTRAN to UTRAN/GERAN as specified in 3GPP TS 23.216. In order to complete the handover of a voice conversation, the voice call needs to be anchored in IMS, such as on SCC AS (Service Centralization and Continuity Application Server), in advance. Steps in FIG. 2 are explained as follows.
Step 1: Local UE founds that signals of the current radio network are not good after a detection, and transmits a measurement report to E-UTRAN.
Step 2: Based on UE's measurement report, source E-UTRAN (specifically, the eNodeB therein) decides to trigger a SRVCC procedure of handing over to UTRAN.
Step 3: Source E-UTRAN transmits Handover Require message to a source MME. SRVCC HO parameter in this message indicates that the target domain of the handover is CS domain.
Step 4: Based on QCI (QoS Class Identity) of an audio bearer and the SRVCC HO parameter, the source MME separates the audio bearer from non-audio bearers, and initiates the PS to CS handover procedure for the audio bearer only to MSC Server.
Step 5: The source MME transmits SRVCC PS to CS Request message to the MSC Server.
Step 6: When the MSC server is different from the target MSC, this MSC server transmits Prepare Handover Request message to the target MSC.
Step 7: The target MSC transmits Relocation Request message to the target RNS, requesting an allocation of relocated related resources of CS domain. The target RNS gives a corresponding response message.
Step 8: When the MSC server is different from the target MSC, the target MSC transmits Prepare Handover Response message to this MSC server.
Step 9: A circuit connection between the target MSC and MGW is established.
Step 10: The MSC Server triggers the session handover procedure, and transmits INVITE message to IMS.
Step 11: SDP using target CS access leg updates a remote UE (i.e., the counter part of establishing the VoIP session with the local UE). At this time, the downlink of the VoIP conversation is handed over to CS domain.
Step 12: EPC PS access leg is released according to 3GPP TS 23.237.
Step 13: The MSC server transmits SRVCC PS to CS Response message to the source MME.
Step 14: The source MME transmits Handover Command message to the source E-UTRAN.
Step 15: The source E-UTRAN transmits Handover from E-UTRAN Command message to UE.
Step 16: UE hands over to the target UTRAN cell.
Step 17: The target RNS detects the attachment of the UE. UE transmits Handover Complete message to the target MSC via the target RNS (specifically, the RNC therein). When the target MSC is not same with the MSC Server, the target MSC transmits SES (Handover Complete) message to the MSC server.
Step 18: The target RNS transmits Relocation Complete message to the target MSC.
Step 19: When the target MSC and the MSC Server are not the same one, the target MSC transmits SES (Handover Complete) message to the MSC Server.
Step 20: In accordance with 3GPP TS 23.009, the target MSC transmits ANSWER message to the MSC server, indicating that a circuit link between the MSC Server and the target MSC has been established.
Step 21: The MSC Server transmits SRVCC PS to CS Complete Notification message to the source MME, and accordingly, the source MME responds to this message.
Step 22: If necessary, the MSC server may transmit MAP Update Location message to HSS/HLR. The MSC Server performs this step to get circuit domain value-added service information.
However, the SRVCC solution specified in 3GPP TS 23.216 is only for audio service, without giving a solution for the handover of video service from PS domain to CS domain for maintaining the continuity thereof.
3GPP TS 23.237 specifies a solution of handovers from PS domain to CS domain for both audio and video services, this solution however only supports Dual Radio UE, i.e., a UE capable of receiving or transmitting data via two types of access networks concurrently, but not Single Radio UE, i.e., a UE can only transmit or receive data only via one of two types of access networks at a moment.
In the increasingly competitive telecommunications industry, service is becoming more diverse and humane, and the requirement for being able to provide video conversation continuity supporting Single Radio is gradually increased, which obviously can not be met by various existing solutions.